Engine systems may be configured with a boosting device, such as a turbocharger, for providing a boosted aircharge and improving peak power outputs. Therein a turbine is rotated using energy from an exhaust flow, the turbine then driving a compressor which delivers a boosted aircharge to the engine intake. In an effort to meet stringent federal government fuel economy standards, engine systems may also be configured with exhaust gas recirculation (EGR) systems wherein at least a portion of the exhaust gas is recirculated to the engine intake. For example, the EGR system may be a low-pressure EGR system (LP-EGR) that recirculates exhaust gas from downstream of an exhaust turbine to upstream of an intake compressor. EGR benefits include an increase in engine dilution and improvements in fuel economy.
However, since EGR has a relatively large water content, the addition of LP-EGR to the intake at a pre-compressor location increases the risk of condensation at both a compressor inlet as well as a charge air cooler outlet. Specifically, under cold ambient conditions, when the humid EGR is mixed with cold ambient air, water droplets can form. Water droplets impacting the compressor blades which are rotating at high speeds (e.g., 200,000 rpm or above) can cause damage to the blades. In addition, since the ingested water slows the rate of combustion, the introduction of water into the engine can increase the likelihood of misfire events.
To address these issues, engine control systems may employ various approaches to limit the condensation. One example approach is shown by Joergl et al. in US publication 2009/0071150. Therein, a mixing pipe is located inside an intake tube for receiving EGR. To reduce damage to the compressor wheel, fresh air is mixed with EGR inside the mixing pipe before the mixture is delivered onto the compressor wheel in an area of low circumferential speed. Another example approach is shown by Clarke et al in U.S. Pat. No. 8,286,616. Therein, an amount of exhaust recirculated via an LP-EGR system and an EGR passage coupled between the exhaust manifold and the intake manifold is adjusted responsive to the humidity in the combustion chamber so as bring the estimated humidity to a desired range.
However, the inventors herein have identified issues with such approaches. For example, the approach of '150 requires an additional mixing pipe which can add component cost and complexity. As another example, condensation may occur at a compressor even when EGR is adjusted based on combustion chamber humidity due to variations in ambient humidity and ambient temperature causing fluctuation in conditions at the compressor inlet. As a result, even with the above mentioned mixing approaches, condensation can occur, leading to compressor degradation.
In one example, some of the above issues may be addressed by a method for a boosted engine comprising adjusting a relative amount of compressed air recirculated to upstream of a compressor from a first passage downstream of the compressor and downstream of an intercooler, and from a second passage downstream of the compressor and upstream of the intercooler based on compressor inlet temperature. The adjusting may be further based on EGR. In this way, a temperature and humidity controlled mixture of compressor recirculation flow and EGR can be delivered to the engine to provide engine dilution while reducing the risk of condensation.
As an example, a boosted engine system may include a turbocharger having a compressor driven by a turbine, and a charge air cooler coupled downstream of the compressor for cooling boosted air before delivery to an engine intake. At least a first recirculation passage may be provided for recirculating cooler boosted air from downstream of the charge air cooler to a location upstream of the compressor inlet. Additionally, a second recirculation passage may be provided for recirculating warmer boosted air from upstream of the charge air cooler to the location upstream of the compressor inlet. In one example, each recirculation passage may have a dedicated valve. Alternatively, the two recirculation passages may merge at a position upstream of the compressor inlet, and a common recirculation valve may be used. The recirculation valve(s) may be a continuously variable valve whose position is adjustable to any position from a fully open position to a fully closed position.
During conditions when low pressure EGR is requested, an engine controller may adjust the opening of the valve(s) to provide a temperature-controlled mixture upstream of the compressor inlet. A proportion of cooled compressed air recirculated from a post-cooler location relative to a proportion of warm compressed air recirculated from a pre-cooler location may be adjusted to maintain the compressor inlet above a threshold temperature, below which condensation at the compressor is likely. For example, in response to an increase in ambient humidity, and/or an increase in the water content in the EGR, the proportion of warm compressor recirculation flow is increased (and the proportion of cold compressor recirculation flow is decreased). The temperature-controlled compressor recirculation flow is then mixed with the low pressure EGR upstream of the compressor and the mixture is delivered to the compressor inlet. The opening of the valve may be adjusted based on a difference between an estimated compressor inlet temperature and a desired compressor inlet temperature such that proportion of warm compressor recirculation flow is increased as the estimated compressor inlet temperature falls below the desired compressor inlet temperature. In alternate examples, such as when ambient temperature is higher, the valve opening may be adjusted to decrease the proportion of warm compressor recirculation flow and increase the proportion of cold compressor recirculation flow.
In this way, by recirculating warmer boosted air from upstream of a charge air cooler to a compressor inlet when condensation is possible, a temperature-controlled mixture may be provided at the compressor inlet using existing engine hardware. By warming the compressor inlet using the compressor recirculation flow, condensation at the compressor inlet is reduced even as ambient conditions or EGR conditions fluctuate. By reducing the amount of EGR condensation ingested at a compressor, condensation related combustion issues may be reduced. For example, misfires may be reduced. In addition, degradation of compressor components due to impingement of condensate on the compressor blades at high rotation speeds is reduced. Overall, the performance and life of the compressor is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.